The present invention relates to a color cathode-ray tube having an in-line type electron gun structured so as to emit three electron beams in a horizontal plane toward a phosphor screen.
Generally, recent color cathode-ray tubes employ an in-line type electron gun. The in-line type electron gun is structured to emit a plurality of, usually three, electron beams in a common plane (horizontal plane). The plurality of electron beams are focused on a phosphor screen of the color cathode-ray tube to reproduce a color image.
FIG. 3 depicts an axially cross-sectioned view illustrating a prior color cathode-ray tube having an in-line type electron gun. The color cathode-ray tube is made up of a panel 10, a funnel 20, a neck 30, a phosphor screen 40 formed on the inner surface of the panel 10, a shadow mask 50 that is a color selection electrode, and a deflection yoke 60 mounted outside the funnel 20. The in-line type electron gun 70 (hereinafter referred to as the electron gun) is contained in the neck 30. In the figure, R, G, and B denote a red, green, and blue electron beams, respectively.
The three electron beams R, G, and B emitted from the in-line type electron gun 70 are deflected by the deflection yoke 60 horizontally and vertically. The electron beams then are color-selected by the shadow mask 50 and impinge on and excite the phosphor screen 40 of the intended color corresponding to each beam to reproduce a two-dimensional image.
FIG. 4 depicts a vertical cross-sectioned view illustrating a prior in-line type electron gun. The electron gun is made up of a cathode 01, a first electrode 02 (hereinafter referred to as the G1 electrode), a second electrode 03 (G2 electrode), a third electrode 04 (G3 electrode), a fourth electrode 05 (G4 electrode), a fifth electrode 06 (G5 electrode), a sixth electrode 07 (G6 electrode), an aperture 08 of the G1 electrode, an aperture 09 of the G2 electrode, an aperture 010 of the G3 electrode on the G2 electrode side, an opening 011 of the G3 electrode on the G4 electrode side, an opening 012 of the G4 electrode, an opening 013 of the G5 electrode on the G4 electrode side, an opening 014 of the G5 electrode on the G6 electrode side, and an opening 015 of the G6 electrode.
A diameter of the aperture 08 of the G1 electrode 02 is 0.4 to 0.6 mm. A diameter of the aperture 09 of the G2 electrode 03 also is 0.4 to 0.6 mm. The opening 011 of the G3 electrode 04 on the G4 electrode side is around 4.0 mm in diameter. The opening 012 of the G4 electrode 05 also is around 4.0 mm in diameter. The opening 013 of the G5 electrode 06 on the G4 electrode 05 side also is around 4.0 mm in diameter. An axial length of the G4 electrode 05 is 1.0 mm. An axial length of the G5 electrode 06 is 17.3 mm.
The in-line type electron gun structured as described above operates as follows.
Thermoelectrons emitted by the cathode 01 heated by heaters are attracted toward the G1 electrode 02 by a positive voltage of 400 to 1,000 V applied to the G2 electrode 03 to form the three electron beams arranged in a plane perpendicular to the sheet of drawing.
Each of the three electron beams passes through the aperture 08 of the G1 electrode 02 and passes the aperture 09 of the G2 electrode 03. The beam then is preliminarily focused a little by a sub-main lens formed of the G3 electrode 04 having a low voltage of around 5 to 10 kV applied thereto, the G4 electrode 05 having the same voltage applied thereto as a voltage impressed on the G2 electrode 03, and the G5 electrode 06 having the same voltage applied thereto as the voltage of the G3 electrode 04. The sub-main lens is formed of a lens between the G3 electrode 04 and the G4 electrode 05 and a lens between the G4 electrode 05 and the G5 electrode 06. The beam, in turn, is accelerated by a positive voltage applied to the G5 electrode 06, and enters a main lens formed between the G5 electrode 06 and the G6 electrode 07.
A potential difference between the G5 electrode 06 and the G6 electrode 07 with a high voltage of around 20 to 35 kV applied thereto constituting the main lens forms an electrostatic field between the G5 electrode 06 and the G6 electrode 07. Trajectories of the three electron beams fed into the main lens are bent by the electrostatic field.
As a result, each of the three electron beams is focused on the phosphor screen to form a beam spot.
To prevent defocusing of the beam spot at the periphery of the screen, the Japanese Patent Publication No. 53-18866 discloses a color cathode-ray tube having an in-line type electron gun having a rectangular recess elongated horizontally and superposed on the aperture 09 of the G2 electrode 03, on the G3 electrode 04 side.
FIG. 5 depicts a plan view illustrating the G2 electrode having the rectangular recesses elongated horizontally and superposed on the aperture of the G2 electrode 03 on the G3 electrode side. The horizontally elongated rectangular recesses 9a, 9b, and 9c enclose the three respective apertures 9.sub.1, 9.sub.2, and 9.sub.3 aligned in line in the G2 electrode 03, on the G3 electrode side.
An appropriate depth in electrode-thickness direction of the rectangular recesses 9a, 9b, and 9c provides electron beams with an appropriate astigmatism to cancel aberrations due to deflection.